This invention relates to gas analysis, and more particularly, to a method or process for determining the nitric oxide content of a gas stream and also to a scrubber apparatus which is capable of selectively removing nitrogen dioxide from a gas stream without affecting the nitric oxide content thereof.
Nitric oxide is a common irritating component found in polluted air. In order that levels of this constituent may be known, means are required for practically and inexpensively determining the level of nitric oxide in air. While the description of the present invention relates particularly to the determination of nitric oxide in air, it is to be understood that the invention is applicable to the determination of nitric oxide in any gas stream which also contains nitrogen dioxide. The two gases, nitric oxide and nitrogen dioxide, are usually found together in polluted air and in other gas streams.
There are a variety of analytical instruments and analytical techniques available today for determining the content of certain irritating gaseous species in gas streams such as polluted air. One such instrument is described in U.S. Pat. No. 3,314,864 to Hersch, the description of which is incorporated by reference herein. How such an instrument is used to analyze nitric oxide is described in commonly owned U.S. Pat. 3,652,227 issued Mar. 28, 1972. Another analytical procedure and the necessary apparatus therefor is described in the U.S. Department of Health, Education and Welfare Public Health Service publication No. 999-AP-111, Selected Methods for the Measurement of Air Pollutants in a chapter entitled "Determination of Nitrogen Dioxide and Nitric Oxide: Saltzman Method" with specific reference to pages C-1 through C-7 inclusive, which is also incorporated by reference herein.
In the technique utilizing a Hersch cell, the nitric oxide is converted by reaction with ozone to form nitrogen dioxide which gives a reading in the instrument. Accordingly, one gas must be removed from the gas stream before the other can be measured. Similarly, in the Saltzman technique, the nitric oxide is first converted to nitrogen dioxide and then the amount of nitrogen dioxide determined. Here again, the original nitrogen dioxide must be removed from the stream before the nitric oxide can be determined as nitrogen dioxide.
In both cases, as is true of other analytical techniques and instruments, it is desired the nitrogen dioxide be removed substantially completely in a scrubber through which the nitric oxide will pass substantially unattenuated. Activated carbon has been tried as a scrubber material. Activated carbon does remove nitrogen dioxide but it also reduces the level of nitric oxide in the gas stream. A reduction of nitrogen dioxide to nitric oxide is observed with a ferrous sulfate scrubber material. In other words, the scrubber serves as a source of nitric oxide when it has been exposed to nitrogen dioxide. This is a significant problem for use with a nitric oxide analyzer which is to be specific for nitric oxide which is present, as is so frequently the case, in a gas stream containing both nitric oxide and nitrogen dioxide.